1. Field of the Invention
The present invention relates generally to engines powered at least partially by a gaseous fuel such as natural gas (hereafter described as xe2x80x9cgas-fueled enginesxe2x80x9d) and, more specifically, to a gas-fueled engine which is of the compression ignition type and which incorporates measures to inject a pilot fuel into a combustion chamber of the engine during its compression stroke, thereby permitting ignition of the gaseous fuel charge by compression ignition. The invention additionally relates to a method for maximizing the pilot fuel ignition intensity in a gas-fueled, compression ignition engine.
2. Discussion of the Related Art
Recent years have seen an increased demand for the use of gaseous fuels as a primary fuel source in compression ignition engines. Gaseous fuels such as propane or natural gas are considered by many to be superior to diesel fuel and the like because gaseous fuels are generally less expensive, and when used in compression ignition engines, provide equal or greater power with equal or better fuel economy, and produce significantly lower emissions. This last benefit renders gaseous fuels particularly attractive because recently enacted and pending worldwide regulations may tend to prohibit the use of diesel fuel as the primary fuel source in many engines. The attractiveness of gaseous fuels is further enhanced by the fact that existing compression ignition engine designs can be readily adapted to burn these gaseous fuels.
One drawback of gaseous fuels is that they exhibit significantly higher ignition threshold temperatures than do diesel fuel, lubricating oil, and other liquid fuels traditionally used in compression ignition engines. The compression temperature of the gas and air mixture is insufficient during operation of standard compression ignition engines for autoignition. This problem can be overcome by igniting the gaseous fuel with a spark plug or the like. It can also be overcome by injecting limited quantities of a pilot fuel, typically diesel fuel, into each combustion chamber of the engine in the presence of a homogenous gaseous fuel/air mixture. The pilot fuel ignites after injection and burns at a high enough temperature to ignite the gaseous fuel charge. Pilot-ignited, compression ignition, gas-fueled engines are sometimes called xe2x80x9cdual fuelxe2x80x9d engines, particularly if they are configured to run either on diesel fuel alone or on a combination of diesel fuel and a gaseous fuel. They are often sometimes referred to as MicroPilot(copyright) engines (MicroPilot is a registered trademark of Clean Air Partners, Inc. of San Diego, Calif.), particularly if the pilot fuel injectors are too small to permit the use of the engine in diesel-only mode. The typical true xe2x80x9cdual fuelxe2x80x9d engine uses a pilot charge of 6 to 10% of maximum fuel rate. This percentage of pilot fuel can be reduced to 1% of maximum, or even less, in a MicroPilot(copyright) engine. The invention is application to true dual fuel engines, MicroPilot(copyright) engines, and other pilot-ignited, compression ignition, gas-fueled engines as well. It will be referred to simply as a xe2x80x9cdual fuel enginexe2x80x9d for the sake of convenience.
A disadvantage of dual fuel engines over spark-ignited engines is the potential generation of increased quantities of oxides of Nitrogen (NOX) resulting from sub-maximum ignition intensity of the pilot fuel charge and resultant less than optimal combustion of the pilot and gas fuel charges. The inventors theorize that less than maximum ignition intensity results from failing to time pilot fuel autoignition to at least generally occur after optimal penetration, distribution, and vaporization of the pilot fuel charge in the gas/air mixture. If autoignition (defined as the timing of initiation of pilot fuel combustion) occurs too soon after pilot fuel injection, the pilot fuel will be heavily concentrated near the injector because it has not yet time to spread throughout the combustion chamber. As a result, overly rich air/fuel mixtures are combusted near the injector, while overly lean mixtures are combusted away from the injector. Conversely, if autoignition occurs too long after pilot fuel injection, excessive pilot fuel vaporization will occur, resulting in misfire.
Moreover, premixed combustion of the pilot fuel, i.e., combustion occurring after the fuel mixes with air, provides greater ignition intensity than diffusion combustion, i.e., combustion occurring immediately upon injection into the combustion chamber and before the fuel mixes with air. Maximizing premixed combustion of pilot fuel is enhanced by retarding autoignition to give the pilot fuel an opportunity to thoroughly mix with the air and form a homogeneous gas/pilot/air mixture. However, retarding autoignition timing is usually considered undesirable in diesel engine technology. In fact, it is almost universally agreed that optimum combustion in a conventional compression ignition diesel engine is achieved with the shortest possible ignition delay, and it is generally preferred that the ignition delay period should always be much shorter than the injection duration in order avoid an excessive rate of pressure rise, high peak pressure, and excessive NOX emissions. (See, e.g., SAE, Paper No. 870344, Factors That Affect BSFC and Emissions for Diesel Engines: Part II Experimental Confirmation of Concepts Presented in Part I, page 15). Conventional dual fuel engines, however, do not allow sufficient mixing time to maximize ignition intensity by igniting a pilot charge that is largely pre-mixed.
The need has therefore arisen to maximize the ignition intensity of a dual fuel charge.
It has been discovered that the relationship between ignition delay and injection duration is an important consideration when pilot injection is optimized for achieving the most intense ignition. The best performance is achieved when the fuel and combustion environment are controlled such that the duration of injection of pilot fuel is less than the ignition delay period (defined as the time between start of pilot fuel injection and the start of pilot fuel autoignition). Stated another way, the best performance is obtained when the ratio Dp/Di less than 1, where Dp is the injection period and Di is the ignition delay period. It is believed that the pilot spray becomes thoroughly pre-mixed during the mixing period Dm occurring between the end of pilot fuel injection and the beginning of autoignition, Ti. This thorough premixing leads to maximized ignition intensity and dramatically reduced emissions. Hence, the inventors have surprisingly discovered that improved results stem from proceeding directly away from the conventional wisdom of providing an ignition delay period that is shorter than the injection duration period. However, in the preferred embodiment, the mixing period Dm preferably should be controlled to also be sufficiently short to avoid misfire.
The ratio Dp/Di can be varied by varying pilot fuel injection timing, pilot fuel injection duration, or autoignition timing. Because Dp/Di is dependent on ignition delay, the ratio Dp/Di can be optimized for a given Di by determining an optimum Dm and adjusting engine operating parameter(s) as necessary to obtain the determined optimum Dm. This control is preferably performed on a full time, full speed and load range basis. It may be either open loop or closed loop.
Ignition intensity maximization can also be thought of in terms of the peak power generated by the pilot ignition. If injection and autoignition are controlled to maximize the number and distribution of pilot fuel droplets and to minimize their size, ignition power on the order of 100 kW/l is obtainable, resulting in extremely effective ignition of the gaseous fuel charge. Ignition under these circumstances can be considered analogous to the simultaneous energization of tens of thousands of tiny spark plugs distributed throughout the gas/fuel mixture.